The present invention relates to a novel photo-reflective sensor, and more particularly to improvements in a photo-reflective sensor in which the light emitted from a light-emitting element is modulated by a variation in the reflectance of an article to be detected and the modulated reflected light is received by a light-detecting element.
Photoelectric converter elements are used in recent years for detecting values which alter from moment to moment, for example, for controlling the number of revolutions of motors or for use in speed alarms for motor vehicles. These sensors are divided into two types: photo-interrupter sensors which comprise a light-emitting element and a light-detecting element opposed thereto to provide a light path so that an article to be detected, when blocking the light path, is detected by the light-detecting element, and photo-reflective sensors which comprise a light-emitting element and a light-detecting element disposed in the same direction as the light-emitting element and adapted to receive the light emitted therefrom as modulated by and reflected from an article to be detected. Because the photo-reflective sensor has a simple construction and can be compacted, sensors of this type have found wide use in peripheral units of electronic computers, such as card readers and readers for characters and diagrams, tacho-generators and control units for controlling the speed of rotation of motors, for example, for sound systems and tape recorders.
FIGS. 1 and 2 show such photo-reflective sensors. The sensor shown in FIGS. 1 (a) and 1 (b) includes a light-emitting element 1 and a light-detecting element 2. The light-emitting element 1 is produced by preparing a header 8 having an upper recess in the form of an inverted truncated pyramid, fixing a lead wires 6 to the header 8, attaching a light-emitting chip 3 to the lead wire 6 by die-bonding (i.e. bonding at an elevated temperature with use of a eutectic solder, an electroconductive epoxy resin adhesive or the like), connecting an internal lead wire 7 to the bonding pad of the chip 3 with a fine gold wire 5 by wire-bonding (i.e. bonding with application of heat and pressure, or ultrasonic bonding), filling a transparent resin 10 in the recess for protecting the chip 3 and gold wire 5 and solidifying the resin 10. The light-detecting element 2, comprising a light-detecting chip 4, is fabricated in the same manner as the element 1. The elements 1 and 2 are accommodated in a container 9 and bonded thereto with a synthetic resin.
The sensor shown in FIGS. 2 (a) and 2 (b) is fabricated by attaching a light-emitting chip 3 and a light-detecting chip 4 to a header 11, for example, of the TO-72 type having four lead wires 6 and 7 with the die-bonding of the chips to the wires 6 and the wire-bonding of the chips to the wires 7, fixing a case 13 having a light-blocking wall 12 to the header 11 as seen in FIG. 2 (a), filling the recesses of the case 13 with a transparent synthetic resin 10 and solidifying the resin as seen in FIG. 2 (b).
The photo-reflective sensor shown in FIG. 1 (b) requires an increased number of assembly steps and is therefore costly, since the light-emitting element 1 and the light-detecting element 2 are produced individually and the elements 1 and 2 are thereafter installed in the container 9. Moreover there is a limitation on the compaction of the sensor, while the difficulty encountered in positioning the elements 1 and 2 impairs the characteristics of the sensor.
The sensor shown in FIG. 2 (b) in which the header 11 and the case 13 are not integral similarly requires an increased number of assembly steps and is costly to make, while failing to have the desired light-blocking properties due to the presence of a clearance between the header and the case. Consequently the sensor has poor characteristics. Further with the construction of FIG. 2 (b) in which the case 13 is attached to the separate header 11, it is very difficult to diminish the diameter of the sensor to less than 5.45 mm. This imposes a limitation on compaction.
With the recent trend that electric parts are incorporated into integrated circuits with an ever increasing density, there is a tendency to provide electric devices in smaller sizes as well as the compaction of motors. It is nevertheless difficult to use conventional photo-reflective sensors for controlling the speed of compacted motors of reduced size because minimum diameter of the sensors is 5.45 mm. and can not be made smaller. Thus it is impossible to incorporate such a sensor into a small-sized motor having a clearance of only about 3 mm., for example.